It is known in the prior art to use differential pressure measurements to determine the level of fluid in tanks and vessels. A conventional practice for using differential pressure measurements is to attach a differential pressure transducer near the bottom of a vessel and to expose one side of the differential pressure transducer to the fluid in the vessel. To account for the static vapor pressure in the vessel, a remote seal is positioned near the top of the tank. The remote seal directly exposes the other side of the differential pressure transducer to the static vapor pressure in the vessel. This arrangement is illustrated in the prior art system shown in FIG. 1.
Such prior art arrangements have encountered a number of problems that prevent the measurement of liquid levels in vessels or result in highly inaccurate measurements. A first problem is that such arrangements are sensitive to temperature changes. A second problem is that a remote seal may be susceptible to plugging or clogging. A third problem is hydrogen migration or permeation when such arrangements are used with vessels containing hydrogen. A fourth problem is condensation within the measurement apparatus. A fifth problem is leakage of fill-fluid used in measurement apparatus.
FIG. 1 illustrates a system 10 with a differential pressure transducer 12 located near the bottom of a vessel 14 and a remote seal 16 located near the top of the vessel 14. The system 10 includes an external fill-fluid line 18 extending from the differential transducer 12 to the remote seal 16. The vessel 14 is designed to store fluid and normally includes vapor or other such gases occupying the space 20 above the fluid level 22. In the arrangement illustrated in FIG. 1, the pressure produced by such vapor, i.e., the static vapor pressure of the vessel 14, is directly accessed by the remote seal 16 coupled to the vessel 14 near the top of the vessel 14. The fill-fluid line 18 provides a fluid path between the remote seal 16 and the differential pressure transducer 12.
The static vapor pressure is transferred to the differential pressure transducer 12 by applying a force to fill-fluid in the fill-fluid line 18 through the remote seal 16. The pressure produces a force on the fill-fluid, which in turn produces a force on the differential pressure transducer 12.
The system 10 remains sensitive to temperature changes because the remote seal is a closed hydraulic system. If the system 10 experiences a drop in temperature, the fill-fluid contracts and when the temperature increases, the fill fluid expands. Unless the diaphragm at the remote seal is very compliant, significant error is introduced to the measurements. Under such conditions, the pressure sensed by the differential pressure transducer 12 due to the static vapor pressure of the vessel 14 will not be reflective of the actual static vapor pressure in the vessel 14 nor the actual liquid level.
When the system 10 of FIG. 1 is exposed to a cold environment, the viscosity of the fill-fluid may cause a failure of the system 10. The viscosity of the fill-fluid may become high enough to clog the fill-fluid line 18. In such a state, the fill-fluid will not transmit forces due to the static vapor pressure of the vessel 14 to the differential pressure transducer 12.
If the vessel includes a hydrogen-rich fluid, hydrogen will dissolve into the fill-fluid. The addition of hydrogen to the fill-fluid may change the density of the fill-fluid, thus decreasing the accuracy of any measurements. In addition, as the static vapor pressure in the vessel 14 is lessened, the hydrogen in the fill-fluid will expand and bubble out of the fill-fluid, which also decreases accuracy of measurements and may cause the diaphragm to burst.
The accuracy of the system 10 may also be compromised due to very small leaks of fill fluid from the remote seal 16 or fill fluid line 18. Any of these occurrences will jeopardize the accuracy of any measurements produced by the system 10.
As the prior art includes a number of drawbacks, there is a need for systems and methods for accurately determining the level of fluid in a vessel.